Calculate how many rolls of roof underlayment you need. Accounts for coverage per roll, overlap loss, and waste for felt or synthetic underlayment.
Roof underlayment is a critical secondary weather barrier installed directly on the roof deck before shingles, tiles, or metal roofing. Whether you use traditional #15 or #30 felt, or modern synthetic underlayment, you need to account for overlap loss, which reduces the effective coverage per roll.
This calculator takes your total roof area and the underlayment product's specifications (roll coverage and overlap) to determine how many rolls to order. Synthetic underlayment typically covers 1,000 sq ft per roll with 3–4 inches of overlap, while #15 felt covers 400 sq ft with 2–4 inches of horizontal overlap.
Ordering the right number of rolls prevents mid-project trips to the supply house and ensures continuous coverage without gaps that could allow water intrusion during construction.
Precise calculations are essential for meeting regulatory requirements, passing inspections, and ensuring the long-term structural integrity and safety of the completed project. This data-driven approach helps contractors minimize rework, avoid delays caused by material shortages, and deliver projects on time and within the agreed budget.
Roll coverage on the label assumes zero overlap. In reality, you lose 10–20% of coverage to horizontal and vertical overlaps. This calculator adjusts for real-world overlap loss to give an accurate roll count. Consistent use of this tool across projects builds a library of reference data that improves estimating accuracy over time and reduces reliance on individual experience alone.
Effective Coverage = Roll Coverage × (1 − overlap%/100) Rolls = Total Area / Effective Coverage
Result: 3 rolls
Effective coverage = 1,000 × (1 − 0.12) = 880 sq ft per roll. Rolls needed = 2,500 / 880 = 2.84, round up to 3 rolls.
Traditional asphalt felt paper has been used for over a century but is being rapidly replaced by synthetic underlayment. Synthetic products made from polypropylene or polyethylene are stronger, lighter, and provide better traction for workers on steep roofs. Price per square is often comparable, making synthetic the better choice for most installations.
Horizontal (side) overlaps should be at least 4 inches. Vertical (end) overlaps should be 6 inches minimum. At hips and valleys, underlayment layers should overlap at least 6 inches past the hip or valley line. These overlaps are where the 10–20% coverage loss comes from.
In cold climates, the first 3 feet from the eave should be covered with self-adhering ice and water shield membrane, not standard underlayment. This prevents ice dam damage. See the separate ice and water shield calculator for that specific material.
#15 felt weighs about 15 lbs per 100 sq ft and is used under shingles on steep roofs. #30 felt weighs about 30 lbs per 100 sq ft, is thicker and more durable, and is required under tile, slate, and on lower-slope roofs. Many codes now require #30 or synthetic as minimum.
#15 felt: approximately 400 sq ft per roll (4 squares). #30 felt: approximately 200 sq ft per roll (2 squares). Synthetic: 500–1,000 sq ft per roll depending on the product and width.
Yes for most applications. Synthetic is lighter (easier to carry up a ladder), more durable, resists tearing and wrinkling, provides better traction on wet surfaces, and has a longer UV exposure tolerance. The per-square cost difference is minor relative to the total roof cost.
Yes. Underlayment under metal roofing provides a secondary moisture barrier and reduces noise. Use synthetic underlayment rated for use under metal (high-temp rated if the metal gets hot).
#15 and #30 felt should not be left exposed for more than 30 days. Synthetic underlayment can typically be exposed for 60–180 days depending on the product (check the spec sheet). Extended UV exposure degrades all underlayment types.
Yes, code often requires double-layer #15 felt on slopes below 4:12 and in ice dam regions. Two layers of #15 felt or one layer of #30 felt provide equivalent protection in most applications.